1. Field of the Invention
The invention generally relates to a touch sensing device, and more particularly, to an optical touch sensing device and a method thereof for preventing an inadvertent palm touch.
2. Description of Related Art
In order for electronic devices to achieve a light, thin, short and compact appearance while allowing users to operate proper functions of the electronic devices easily, touch screens gradually become essential output and input tools for the electronic devices. When operating a touch panel on a portable electronic device (such as a mobile phone or a tablet PC), a user generally would operate the touch panel in a hand dangling manner so as to avoid the palm or other body parts from inadvertently touching the touch panel. Nevertheless, as the appearance of the electronic device becomes increasingly light and thin, a space for the user to hold the electronic device becomes smaller, and thus the user's palm may inevitably touching the touch panel and cause an inadvertent touch or misconduct.
According to working principles of a sensor, the touch panel can generally be classified into various types, such as capacitive, resistive, optical and etc. In a capacitive touch device and a resistive touch devices, there already exist technologies for determining an inadvertent palm touch through a touch area of a touch object. However, an optical touch device is different from the other touch devices in terms of sensor working principles, and is difficult to determine an inadvertent palm touch simply through the size of a covering region of a touch signal. This is because that an area occupied by an image captured by an image capturing unit in the optical touch sensing device would become smaller as a distance from the touch object to a lens of the image capturing unit becomes farther.
FIG. 1A and FIG. 1B are schematic diagrams illustrating an optical touch sensing device 100 in sensing a touch object 120 to generate a lens image 150. A distance D1 of the touch object 120 relative to an image capturing unit 210 in FIG. 1A is smaller than a distance D2 of the touch object 120 relative to the image capturing unit 210 in FIG. 1B, and the touch object 120 shown in FIG. 1A and FIG. 1B has the same size. Referring to FIG. 1A and FIG. 1B at the same time, when the image capturing unit 110 captures the lens image 150 along the touch surface 130, a covering region A1 formed by the touch object 120 in FIG. 1A would be greater than a covering region A2 formed by the touch object 120 in FIG. 1B. Since it is difficult for the optical touch sensing technology to directly use the covering region on the lens image as a judgment basis for an inadvertent palm touch, manufacturers need to find other technique to solve the problem regarding the inadvertent palm touch in the optical touch sensing device.